This invention relates to a thin-film coupler with a carrier substrate and two strip lines disposed thereon, of which one is the main coupler loop and the other the auxiliary coupler loop.
Couplers are inter alia high-frequency components for mobile telephones or base stations which render possible the coupling of HF signals between the output of a power amplifier and an antenna. The signal coupled out is used for controlling the output power of the amplifier. Such a coupler comprises, for example, two coupler loops of which one loop is the main loop which is to transmit the transmission signal with the lowest possible losses. The second loop, the auxiliary loop, couples out a signal which is small compared with the transmission signal.
Such couplers are known in various embodiments. One of these is a coupler in the ceramic multilayer technology. The electrode structures are printed on ceramic foils, the foils are stacked, and subsequently sintered so as to form components in the case of these ceramic couplers. The disadvantage in this printing method is the coarse-grained morphology of the electrodes, which leads to a higher electrical resistance.
Furthermore, there are embodiments in the microstrip technology. A thin-film coupler is described in 1991 IEEE MTT-S International Microwave Symposium Digest, vol. II, 857-860 which comprises two strip lines forming coupler loops. The two coupler loops are provided on a dielectric substrate with a high dielectric constant K. A metal layer is present on the rear side of the ceramic substrate so as to form a grounding plane. Six end contacts are fastened to the component, two of these being in contact with a coupler loop each time, and two being connected to the grounding plane. The use of a dielectric substrate with a dielectric constant has the advantage that the components can be realized in a compact construction. A major disadvantage is, however, that these substrates are substantially more expensive than, for example, glass or Al2O3. If a compact coupler (coupler length  less than  less than xcex/4) is realized on such inexpensive substrates, the couplers will show a frequency shift of the coupler signal of approximately 6 dB/frequency octave.
The invention has for its object to provide an inexpensive compact coupler which exhibits the same coupling at several frequencies, i.e. over a broad frequency range.
This object is achieved by means of a thin-film coupler with a carrier substrate and two strip lines disposed thereon, of which one is the main coupler loop and the other the auxiliary coupler loop, wherein a component is integrated into the auxiliary coupler loop, which component achieves a phase shift of the frequency of the signal coupled out.
Usually, couplers show a strong frequency dependence in their coupling. The incorporation of a component in the auxiliary coupler loop which achieves a phase shift of the frequency of the signal coupled out makes for a greater band width of the coupling.
In a preferred embodiment of the thin-film coupler, the component achieving a phase shift of the frequency of the signal coupled out and integrated into the auxiliary coupler loop is a strip line.
The incorporation of a strip line in the auxiliary coupler loop represents the simplest embodiment of the thin-film coupler, since the strip line can be integrated directly into the auxiliary coupler loop and no additional process step is necessary in the manufacture.
In another preferred embodiment of the thin-film coupler, the component achieving a phase shift in the frequency of the signal coupled out and integrated into the auxiliary coupler loop is a coil.
A coil may be integrated into the auxiliary coupler loop in a simple manner in that it is also implemented in the strip line technology and is accordingly applied in the same process step as the rest of the auxiliary coupler loop. Alternatively, however, a coil may be provided by means of other thin-film techniques and subsequently be electrically contacted with the auxiliary coupler loop.
In a particularly advantageous embodiment of the thin-film coupler, the component achieving a phase shift in the frequency of the signal coupled out and integrated into the auxiliary coupler loop is formed by a coil and a capacitor connected in series or in parallel.
The incorporation of an LC combination results in a particularly strong enlargement of the band width of the coupler.
Preferably, the material used for the carrier substrate is a ceramic material, a ceramic material with a planarizing layer of glass, a glass-ceramic material, or a glass material. A carrier substrate made from these materials can be inexpensively manufactured and the process cost for the relevant components can be kept low.
It is furthermore preferred that each end of a coupler loop is electrically connected to a current supply contact.
Each component can be electrically connected to further components of a circuit by its current supply contacts. Depending on the type of application or type of component mounting, an electroplated SMD end contact or a bump end contact or a contact surface may be used as the current supply contact. The use of SMD end contacts or bump end contacts renders possible the manufacture of discrete components.
It is also preferred that at least one protective layer of an inorganic material and/or an organic material is provided over the thin-film coupler.
The protective layer protects the component against mechanical loads and corrosion caused by moisture.
It is advantageous that a metal layer is provided on the lower side of the carrier substrate.
This metal layer serves as a grounding plane.
In this advantageous embodiment of the thin-film coupler, it is preferred that the metal layer is connected to at least one further current supply contact.
The invention will be explained in more detail below with reference to six Figures and three embodiments. In the Figures: